The invention relates generally to satellite communication systems and, more particularly, to systems capable of providing dual satellite coverage.
In a satellite communication system, significant advantages can be achieved if the same or a similar frequency band is used to provide both an uplink and a downlink between a satellite and a terrestrial subscriber. For example, such an arrangement allows a single antenna structure to be used on the satellite for both uplinks and downlinks, thus reducing weight and increasing payload space on the satellite. However, use of the same or similar frequency band for both the uplink and the downlink will normally require that some form of time separation be implemented between the transmit and receive activity (e.g., time division duplex (TDD) techniques). This is because a transceiver unit is not generally capable of transmitting and receiving at the same time in the same or similar frequency band because of transmitter leakage into the receive channel that obscures or prevents detection of the receive signal. If the transmit frequency range is different from but near the receive frequency range, then it is possible to use filters to allow simultaneous transmission and reception, but such filter arrangements are complex, costly, and bulky and thus are not generally preferred in satellite applications. Therefore, the transmit activity is generally separated in time from the receive activity. By separating transmit and receive activity in time, however, a significant reduction in system capacity occurs because the available bandwidth is effectively being utilized for only a portion of the available time.
To overcome the reduced capacity problem, at least one satellite communication system has been proposed that is capable of simulating full duplex communication between a terrestrial user and a satellite constellation using a dual satellite coverage approach. A description of such a system can be found in co-pending U.S. patent application Ser. No. 09/094,976, filed on Jun. 15, 1998 which is co-owned with the present application and which is hereby incorporated by reference. In a satellite communication system that provides dual satellite coverage, each subscriber that is communicating with the satellite constellation maintains a communications link with two satellites in the constellation simultaneously. By using two satellites for each subscriber, diversity is obtained which enhances communications reliability and robustness in the system.
To simulate full duplex communications, the system described in the above-referenced patent application utilizes a unique time division duplex (TDD) frame structure within each of the two satellites communicating with a given subscriber. The TDD frame structure used in each of the satellites includes separate transmit and receive time slots for use in communicating with subscribers so that neither satellite transmits and receives in the same frequency band during the same time slot. In addition, the TDD frame structure used in one of the two satellites is the complement of the TDD frame structure used in the other satellite (i.e., the transmit and receive time slots are switched in the two structures). Thus, when one of the satellites is transmitting during a particular time slot, the other satellite is receiving, and vice versa. Because the satellite constellation is both transmitting to and receiving from the subscribers during each time slot, a simulated or xe2x80x9cquasixe2x80x9d full duplex arrangement is achieved.
As can be appreciated, to achieve the above-described benefits, a satellite constellation needs to be developed that is capable of providing dual satellite coverage (preferably continuous) over a region of interest. In addition, the constellation should allow each pair of satellites communicating with a common terrestrial subscriber to operate in accordance with the specific complementary TDD frame configuration designed for the system.